Balloon dilatation catheters are used in the treatment of a variety of vascular conditions. Among the more frequent uses for balloon dilatation catheters is in vascular angioplasty of the peripheral and coronary arteries, by which arteries obstructed by plaque (formed by fatty deposits such as cholesterol) are dilated to improve blood flow through the artery. In a typical angioplasty procedure, a balloon dilatation catheter is inserted percutaneously into the patient's arterial system and then is advanced and steered through the patient's arteries until the distal end of the catheter, that carries the balloon, is disposed adjacent the obstruction (stenosis). The balloon end of the catheter then is advanced into the stenosis and, when so placed, is inflated under high pressure, to dilate the artery in the region of stenosis. The catheter typically is used with a small diameter steerable guidewire which is used to guide the catheter to the stenosis. By way of example, such a catheter and guidewire system is disclosed in U.S. Pat. 4,545,390 issued Oct. 8, 1985 (Leary), reference thereto being made for a more complete description of the catheter and guidewire system and its manner of use.
It is desirable, particularly in coronary angioplasty in which the coronary arteries are narrow and tortuous, and in which the stenoses often may be calcified and difficult to dilate, that the catheter and its balloon meet a number of stringent requirements. Among these are that the balloon be capable of folding down to a low profile about the catheter shaft so that the balloon portion of the catheter is more readily insertable through the stenosis. Inability to insert the balloon portion of the catheter into the stenosis is among the more frequent causes of an unsuccessful angioplasty. Also among the important characteristics of the balloon dilatation catheter is that it should be "trackable", that is, it must be able to follow and advance over the guidewire and through the artery even when the artery is highly tortuous with many sharp bends. An additional important characteristic of the balloon is that it should have a high burst strength so that it may dilate hard, calcified stenoses as well as those that require less force for the dilation.
In order to improve the low profile and trackability characteristics of the character in the region of the balloon, efforts have been made to develop dilatation balloons having very thin walls so that the balloon will fold more readily to a low profile about the catheter shaft and also so that the balloon will be more flexible, thus enhancing the ability of the catheter to bend in the region of the balloon, thereby achieving improved trackability. To that end, significant advances have been made in the art. U.S. Pat. 4,490,421 describes the manufacture of dilatation balloons by which balloons may be made having a high burst strength and significantly thinner walls than its predecessors. The procedure was improved further, as described in prior co-pending U.S. patent application Ser. No. 001,759, filed Jan. 9, 1987, now abandoned and the complete application being explicitly incorporated herein, to enable the manufacture of high strength balloons having even thinner, more flexible walls.
Although the foregoing advances in manufacturing thinner walled balloons have significantly improved the catheters, those efforts have been directed at the cylindrical midportion of the balloon. The cones and necks of the balloon, at the ends of the cylindrical midportion, are not as thin as the cylindrical midportion. Each cone is of increasing wall thickness in a direction away from the cylindrical midportion of the balloon and reaches a maximum wall thickness at its juncture with the necks. The wall thickness of the neck is at that maximum value throughout their length. The increased wall thickness of the balloon in the regions of the cones and the necks detracts from the ability of the balloon to collapse to a low profile as well as the ability of the balloon to track along the guidewire along sharp tortuous paths. It would be desirable, therefore, to provide a balloon for a dilatation catheter in which the wall thickness in the cone and neck portions is reduced and, preferably, is not substantially greater than the thickness in the cylindrical midportion of the balloon. It is among the objects of the invention to provide such a balloon and method for its manufacture.
It is among the general objects of the invention to provide improved dilatation balloons and a method for their manufacture which provides superior properties of thin walls, flexibility and high strength as well as full dimensional stability both in storage and when inflated.